The subject invention relates to coating compositions. More particularly, the invention relates to coating compositions which can be formulated to contain substantially no organic solvent of a high volatility.
Many different coating compositions possessing various physical characteristics are known. The coating compositions normally contain a curable film forming resin and a solvent or dispersant. Examples of curable resins include vinyl resins, acrylic resins, epoxy resins, urethane resins, etc. All the polymeric resins have inherent characteristics which make them more suitable than other resins for a particular end use. Various additives or modifiers are also known which can be added to the coating compositions in minor amounts so as to modify the characteristics of the film former. The solvent or dispersant is present for the purpose of thinning the composition to facilitate its application to a substrate and to improve its flow properties. The solvent and dispersant can generally be defined as a chemical which thins a coating but is volatile and therefore is lost to the atmosphere when the composition is applied to a substrate. Suitable solvents and dispersants have included both organic solvents and water.
In recent years, there have been expressed concerns over the presence of solvents, especially organic solvents, in coating compositions. The organic solvents are mainly derived from petroleum products, and thus are expensive and in short supply. Additionally, a potential pollution problem exists with the organic solvents unless elaborate disposal or recovery procedures are used. A still further concern with the use of solvents in the coating compositions is with respect to the need to supply energy to the coated substrate to drive off the solvent. This energy is needed regardless of whether the solvent is an organic solvent or water.
Water-based coating compositions do not have an inherent potential pollution problem. As a result, much activity has been directed towards the formulation of water-based coatings in recent years. However, one of the problems associated with a water-based coating is the fact the polymeric film former usually has present various hydrophilic groups which are water sensitive and can react with water. As a result, films formed from the water-based compositions do not have all the physical characteristics oftentimes desired and needed. Additionally, energy is still needed to drive off the water, special corrosion-resistant equipment must be used and air conditioning systems are usually needed to control humidity.
Ideally, a coating composition would have little or no solvent present with the attendant problems, yet would be fluid enough for conventional fluid application to substrates. Technologies such as described in U.S. Pat. No. 4,022,726, Zabrocki et al, May 10, 1977, wherein a reactive diluent is used, attempt to meet the above objectives. The reactive diluent is said to be capable of thinning the coating composition yet reactive with the film forming resin so as to become a part of the film upon cure. Unfortunately, many of the available "reactive diluents" evaporate from the thin film prior to reacting and thus do not fulfill their objective.
There has now been found a class of materials which can be added to coating compositions. When used in aqueous compositions, the materials act as coalescing aids. Moreover, the addition of the materials at low levels desirably modifies the characteristics of the film former while addition of the materials at a higher level modifies the film and acts as a reactive diluent. That is, the reactive diluent desirably modifies the film former resin and at the same time thins the coating composition without a substantial pollution problem or energy demand. A consequence of using the reactive diluent is a substantially reduced need for the use of conventional organic solvents.
As used herein, all ratios and percents are by weight unless otherwise indicated.